Indian Journal of Engineering & Materials Sciences Vol. 6, April 1999, pp. 66-72

Effect of acidic water on physico-mechanical behaviour of rock

TN Singh, S K Singh, A Mishra, P K Singh & V K Singh Department of Mining Engineering, Institute of Technology, Banaras Hindu University,

Varanasi 221 005, India

Received 22 September 1998; accepted 28 Janllary 1999

The physico-mechanical properties of rocks are important input parameters for designing of mining and civil engineering construction projects. The presence of acidic water in excavated areas ·pose serious problem and makes the excavation process more difficult . In the present study, sandstone rock specimens were tested in the laboratory in dry, wet and acidic environment. The uniaxial compressive, tensile, shear strength, cohesion and angle of internal fricti on were determined in acidic water conditions. This study reveals that the acidic water changes the behaviour and strength of rock. The strength decreases linearly from neutral pH to acidic ·condition. The angle of internal friction increases with decreasing pH values.

The physico-mechanical properties of rock plays significant role in planning and designing of any mining and civil engineering constructional works, which are the basis of economic evaluation of resource recovery and construction industries .

Rock properties are important to exploratory drilling, drilling and blasting, crushing, hauling, production and communition for many reasons. The state of the art about the strength propert ies related to acidic environment has not been reviewed properly in recent years . The presence of acidic water at mine faces in Indian geo-mining conditions very often pose time consuming interruptions in the excavation process. If the water get acidic, it affects most of the mining operations and the efficiency of operation gets retarded in many folds . The drilling rate is also affected. Many of the penetrated strata are chemically sensitive leading to potential drill bit failures .

In Indian coal industry, the acid mine drainage poses serious problem and adversely affects the durability of machines and hamper the productivity. But very little. work has been done particularly in Ind ia to know the impact of acidic water on the physico-mechanical behaviour of rock mass. This work involves not only fluid flow network and fracture diffusion but also the liquid rock interactions.

Rock can be generally identified as an aggregate of minerals. These particles may be held together by the binding material, i.e., matrix , at the grain boundaries and by interlocking of grains. The grain size affects the strength of rocks. It was found that as the grain size .increases, specimen is expected to be

weaker in terms of load per unit area at breakage. Geological discontinuities like fissures, joints and fault zones are prone to intrusion by surface water and/or hot acidic solution from springs. Hence, chemical alteration in such zones is likely to take place. The chemical action of acidic water which enters in the rock from the surface is referred to as chemical weathering. Hot water intruding the voids in rock, normally experienced at great depth, is said to bring about hydrothermal alteration. The apparent structure of rock depends on the combined 10caJ strength of the matrix and the contact area among the grams.

Boozer and Hiller l reported that tbe deformational behaviour of Indiana limestone and Navajo sandstone are affected by fluids which are strongly absorbed on the surface of the grains of these rocks. ll1e predominant effect of these fluids was observed, as decrease in the ultimate strength of sandstone and a decrease in the yield stress in the case of Indiana limestone, which deformed in ducti le manner. In certain cases, it was observed that the type of failure of Indiana limestone was ductile when the specimen was saturated with an inert liquid and brittle when the limestone was saturated with Oleic acid.

The effect of moisture content on the uniaxial compressive strength of quartzitic sandstone is shown in Fig. 1. The uniaxial compressive strength is almost inversely proportional to the surface tension of different acidic liquids with which the specimen is saturated2

. Fig. 2 shows the effect of different liquids on the compressive strength of sandstone3

. This figure

,1

SINGH et al.: PHYSICO-MECHANICAL BEHAVIOUR OF ROCKS 67

40

1Itt ..... r PZOs

CI CL 35 % ~ g. C • ..

OJ • .~ 30 II II • a. e 0

" c; )(

.2 c 25 ~

20L-__ J-__ ~ __ ~~ __ ~ __ -L __ ~

- 0.4 -0.2 0.0 .0.2 .0.4 .0.6

Moisture content. % by wei ght

Fi g. I-Uniaxia~ compressive strength versus moisture content for Quartzitic sandstone2

also shows the importance of pH on the compressi'l(e strength of the silica rock.

Two principal theories have been proposed to explairl the mechanism by which the liquids affect the compressive strength of various rocks. Rehbinder et al.4 proposed that the strength is altered by a change in the surface-free energy of the developing crack or fracture due to absorption of the Iiquid.Westwood5 proposed that the liquid alters dislocation density and mobility around the cracktip as a function of the zeta potential of the liquid-rock system and thus inhibits or enhances crack propagation. There is still a controversy about the merics of the two mechanisms.

The most important external factor affecting shear strength is the magnitude of the effective normal stress (G,J acting across the weak plane filled with water having different pH value. This value of Gil may lie in the range of 0 .1 - 2.0 MN/m2 (1 - 20 kg/cm2) for most of the rocks encountered in solving engineering problems. But for tectonophysics studies involving shear strength of laboratory induced faults, stress levels may range even up to 100 - 2000 MN/m2

(1 - 20 kbt The higher value of normal stress means a higher shear strength.

Fluid pressure influences rock mass stability by slaking and softening the clays along with the discontinuity of surfaces, thereby causing a significant

o Oodtcy! a ... oni". thloridt 6 Sodh,. tarbonate o AI".iniu. chlorid.

pH vclue

Fig. 2-Uniaxial compressive strength versus pH value of immersion fluids forsandstone 3

reduction in the shear strength of the joint and by infilling tension cracks, exerting hydrostatic forces, along the crack surfaces. Swelling of rock mass, excavated undrained may produce strain and so causes failure 7

.

Influence of fluid pressure on cohesive and frictional properties of the discontinuity depends upon the nature of the filling or cementing materials. In many hard rocks and sandy materials, these properties are not significantly altered by water but clays, shales, mudstones and si milar materials exhibit significant changes with changes in moisture content6

,8 .

According to Sharp and MainiQ

, it was found that seepage of water caused by drainage into an engi neering excavation had far reaching consequences in cases where sinking ground water level caused settlement of stmctures founded on overlying clay deposits.

The physico-mechanical properties of rock play an important role il1 any long or short term stability. Several researchers have done extensive work on the different aspects of physico-mechanical properties and have discussed about the fracture mechanism 10.1 I. The effect of acidic water on physico mechanical properties has not attracted much attention earlier. Some of the Indian coal mines where acidic mine water is encountered, the working is difficult: Keeping in view of these important aspects, it was decided to study the effect of different pH values on the physico-mechanical properties of Chunar sandstone in different geo-environmental conditions .

The objective of the present work is to establish a correlation between acidic water and physico mechanical properties of rocks and analyse the fracture mechanism and modes of failure.

68 [NDIAN J. ENG. MATER. SCI., APRIL 1999

Material and Method Rock core-specimens of Chunar sandstone were

prepared from a selected rock blocks. The NX size cylindrical rock cores were prepared with the help of diamond core drilling machine. These core specimens were smoothened on specially modified lathe Il"!achine to avoid parallelism defects. Then, these specimens were dried, weighed and later submerged into acidic water having different pH values from 2 to 7 till full saturation. After ten days it was found that the weight of the saturated specimen became constant, thereby indicating full saturation. The moisture content of the specimens were determined with the help of the following formula :

~-~I Moisture content(%)

~I x 100

where, W, is the weight of the specImen after saturation, and Wd is the weight of the specimen just after oven drying at 105 DC for 24 h.

The physico-mechanical propert ies of Chunar sandstone spec Imens like uniax ial compress Ive strength , Brazilian test for tensile strength , shear strength, triaxial strength etc were determined at different pH values in the laboratory to understand the behaviour of rock and their fracture mechani sm as per ISRM code. Results and Discussion Effect of pH 011 uniaxia l compressive strength

The cylindrical NX sI ze Chunar sandstone 70

60

.. c. ~ 50 L:: c;, c: ~ cii 40 . .. > 'iii on .. a. 30 E 0 '-l .~ >< 20 .. 'c :J

10

specimens were tested to determine the uniaxial compressive strength on universal testing machine. The uniaxial compressive strength of dry specimen was calculated as 79.83 MPa. The mode of failure was conical wedged shaped which was due to the end constraint by the loading platens(Fig 3) .

The other specimens were also tested at pH values 2, 4, 6, and 7 and a graph was plotted between pH values and the corresponding uniaxial compressive strength obtained (Fig. 4). The highest va lue was obtained at pH 7 whereas lowest at pH 2. Fi g. 4 indicates that the strength increases with pH. It was probably due to increase in open spaces among the intragranular bindings. The corrosive nature of the

Fig. 3-Mode of failure of specimen under compre3sion test

y = -0. 1306x3 + 1.4582; + 0.8168x -+- 33.3481 R2 = t

O+---------~--------_+--------~----------r_--------~--------~---------o 2 3 4 5

pH of the acidic water

Fig. 4--Variation of uni axial compressive strength with pH for acidic water (Saturated Chunar Sandstone)

6 7

SINGH et al.: PHYSICO-MECHANICAL BEHAVIOUR OF ROCKS 69

acidic water with sandstone may also be responsible for decrease in the uniaxial compressive strength. The fracture pattern was not changed much due to change in the pH values. In all the cases, initial fracture extension started near the center of the specimens, where the axial stress is greatest and the lateral constraint is least.

Effect of pH on tensile strength

Brazilian test was conducted to determine the tensile strength of Chunar sandstone. In this test, a circular disc is compressed to failure across a diameter (Fig 5). It was found that at pH 2 the tensile

Fig. 5-Tensile failure of specimen under diametrical mode of fracture

strength was 3'.23- MPa which was lowest while that of dry specimen it was 5.06 MPa which was highest. Fig. 6 shows that the tensile strength increases as the pH value increases though the best fit do not follow the linear path exactly. The fracture initiated from the central part of the disc and propagated radially. Two types of cracks were observed in the failed specimen. First diametrical crack which was present in each case but in addition, there were secondary cracks, which are less symmetrical and complete. It was because of the bounce in the platen caused by sudden reduction in resistance to the applied load at failure. It was also observed that the contact area also influence the origin of the cracks. The mineral (Si02) in this case reacted with H2S04 thus decomposing and creating more intergranular spaces filled with liquid, which contributes fOT the decrease in strength of the specimen. It is also possible that liquid influence the surface energy of the specimens and as new surface originates during loading of the specimen, the liquid which wet the surface of the rock invariably decreases the surface energy of the rock specimen and hence the tensi Ie strength of the rock.

Eff("ct of pH on shear strength of the rock

The cylindrical Chunar Sandstone specImen was tested on uni versal testing machine with double shear

4.5,---------------------------------------------------___________________ ~

'" a. ::!: .r:. c;, c: ~ Ui .!! ·iii c: ., I-

4 ·

3.5

3 ·

2.5

2

1.5 .

0.5

y = -O.009X3 + O.143x2 - 0.436x +"3.602 R2 = 1

O~------_+--------~------~--------~--------~------~------~ o 2 3 4 5

pH of acidic water

Fig. 6--Variation of tensile strength with pH for acidic water (Saturated Chunar Sandstone)

6 7

70 INDIAN J. ENG . MATER. SCI. , APRIL 1999

case(Fig. 7). The shear strength of the Chunar sandstone specimen were determined in dry condition and after submerging in solutions of different pH values as in the case of compressive and tensile strength. The shear strength of dry specimen was calculated and found to be 12.71 MPa, whereas lowest value of shear strength was found to be 4.6 1 MPa at pH 2. Fig. 8 shows that with increase in the pH value the shear strength decreases, i.e., as the acidity increases the shear strength decreases . T.e mode of fractl!-re was compression-cum-shear mode.

Fig . 7-Modes of fai lure under shearing of the specimen

Tensi le stresses were also present for some moment during the bending of the specimen under the loading. High shear stress concentration is at the loading edge and hence stress is not uniform or the .fracture plane. This also follows the same trend as the case of the compressive strength and tensile strength.

Effect of pH on Triax ial Strengt h of the Rock

The Chunar sandstone specimens were tested in triaxial stress condition to know the shear behaviour of the rock at different pH value. It was observed that as the pH value decreases, the angle of friction increases . The lowest angle of friction was 44° at pH 7, whereas 47° at pH value 2. It was probably due to corrosion and dissolving of certain grains and interlocking binding materials because of submersion of the rock specimen in acidic water. Hence, the surface of the speci mens pose undulation which increases the peak and residual shear stresses and ultimately increase the resistance and fr iction angle. A graph between pH values and angle of internal friction is given in Fig. 9.

Cohesive strength decreased as the medium gets acidic (Fig. 10) . All the specimen tested in triaxial cell fractured along an oblique shear plane (Fig 11) and thi s shear plane (was oriented at 30°+2° ) with the

12r--------------------------------------------------------------------------,

-;; 0. ~

r=. en c: ~ Vi ~

'" ., J: VI

10 ·

·8

6

4

2 y = 0.0584,' ·0.5673,' + 2.4103, + ,.5j

R2 = 1

o+---·------~--------~--------~--------~--------~~--------+_------~

o 2 3 4 5

pH of acidic water

Fig. 8- Vari ation of shear strength \vith pH for acid ic water (Saturated Chunar Sandstone)

6 7

'0 c: 0

~ ·c .... .... 0 Q)

c;, c: <t

E u ;;: en ~

~

0, c: ~ iii Q)

> VI Q)

~ 0 u

47.5

47

46.5

46

45.5

45

44.5

44 0

250.00

200.00

150.00

100.00

50.00

SINGH el at.: PHYSICO-MECHANICAL BEHAVIOUR OF ROCKS

2 3 4

pH of acidic water

y = -O.0333x3 + OAx2 - 1.9667x + 49.6

R2::: 1

5 6

Fig. 9--Variation of angle of friction with pH of acidic water (Saturated Chunar Sandstone)

Y = 9.81x + 137.34 R2 = 1

7

0.00 +-------t------+-----+------+-----+------If-------l o 2 3 4 5

pH of acidic water

Fig. I C)- Variation of cohesive strength wi th pH for acid ic water (Saturated Chulla r S~lI1d s t o lle)

6 7

71

72 INDIAN 1. ENG. MATER. SCL, APRIL 1999

Fig. II-Modes of fracture after triaxial loading

axial load direction. It was also observed that compressive strength increased as the confining pressure was increased. The rate of increase in the strength of Chunar sandstone is higher at initial confming pressure. As the confining pressure increases further, the corresponding rate of increase in strength continuously drops down.

Conclusions The following important conclusions are drawn

from this study: I The effect of pH value has significant role in the

physico-mechanical behaviour of the rock. 2 The uniaxial compressive strength and tensile

strength decreases as the medium becomes more and more acidic.

3 The angle of friction determined by triaxial tests increases with decreasing pH values.

4 The cohesive strength of Chunar sandstone is

highest at pH 7 and lowest at pH value 2. In all cases, pH values bears a linear refationship

with the physico-mechanical properties bf Chunar sandstone. This study will be helpful in designing of mine structure on surface as well as underground areas in the region where. water gets acidic due to pyritic contaminations.

Acknowledgement One of the authors (VKS) acknowledges SIncere

thanks to CSIR for financial assistance.

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on the deformation behaviour of rocks subjected to triaxial compression. Proc. Fifth US Symp. on Rock Mech. , 1962, 579- 626.

2 Colback P S B & Wiild B L, The influence of moisture contem on compressive strength of rocks, 3rd Can. Rock Mech. Symp., Toronto, 1965,65-63.

3 Street N & Wang F D, Surface potential and rock strength. 1st Congo Int. Soc. Rock Mech., Lisbon., Vol. I, 1966,451-456.

4 Rehbinder P, Schreiner L & Zhijach K, Hardness reducer in drilling(lzv. Akad. Nauk SSR), I 944.

5 Westwood A, J Mater Sci, 9 (1974) 187\. 6 Barton N & Choubey V, Rock Mech, 10(1977) I-54. 7 Vaughan P.R, in Carrington Slope Stability Ellgineering,edited

by Chandler R J, (Ins!. of Civil Eng), 199 \. 8 Hoek E & Bray J, Rock Slope Engineering (The Institution of

Mining and Metallurgy, London), 1st ed 1977. 9 Sharp JC & Maini Y N T, Fundamental consideration on the

hydraulic characteristics of joints in rock, Proc. ISRM. Symp. on Percolation Through Fissured Rock, Stuttgart, IT-F, 1972,15.

10 Frankin J .A, Classification of rock according 10 its mechanical properties, Ph.D. Thesis, London University, London, 1970.

II Honeyborne D B, Weathering process affecting inorganic building materials, Building Research Station, U.K., Internal. Note INIl41/65,1965